1. Field of the Invention
The present invention relates to a semiconductor device. More particularly, it relates to a method for producing bipolar transistors of an integrated circuit (IC).
2. Description of the Prior Art
In order to increase the density of an IC or large-scale integration circuit comprising many bipolar transistors, it is proposed to produce a bipolar transistor by a washed emitter process. According to the washed emitter process, a method for producing a bipolar transistor comprises the steps of selectively etching a silicon oxide layer formed on a base region to form an opening for defining an emitter region, forming an oxide layer containing impurities within the opening and on the silicon oxide layer, diffusing the impurities into the base region to form the emitter region, washing out the impurity-containing oxide layer, and forming an emitter electrode. The washing out of the impurity-containing oxide layer is carried out by using a suitable etching liquid (e.g., hydrofluoric acid) and by using no masking pattern. In the case of the washed emitter process, the opening for emitter diffusion serves as an emitter electrode opening, whereby there is no gap between the position of the emitter electrode opening and that of the emitter region. Accordingly, it is possible to reduce the dimensions of the emitter region to the minimum dimension for etching the silicon oxide layer. Thus, the size of the emitter region is made smaller than the conventional emitter region so that the base region and a silicon well (i.e., a silicon island) can be made smaller than conventional ones. However, although the impurities forming the emitter region diffuse in a horizontal direction during the diffusion step, a short between the emitter and base (i.e., a so-called emitter-base short) easily occurs in the bipolar transistor produced by the washed emitter process since the silicon oxide layer is simultaneously etched more or less to enlarge the opening when washing out the impurity-containing oxide layer.